The present invention relates generally to methods of forming thin films and, more particularly, to methods of forming high dielectric constant films on hydrogen-passivated substrates.
Current Si VLSI technology uses SiO2 as the gate dielectric in MOS devices. As device dimensions continue to scale down, the thickness of the SiO2 layer must also decrease. Thicknesses of less than 1 nanometer (nm) are expected in the future. However, the occurrence of high tunneling current through such thin layers of SiO2 requires that alternate materials be considered. Materials with high dielectric constants would permit gate dielectric layers to be made thicker, and so alleviate the tunneling current problem. These so-called high-k dielectric films are defined herein as having a high dielectric constant relative to silicon dioxide. Typically, silicon dioxide has a dielectric constant of approximately 4, while it would be desirable to use a gate dielectric material with a dielectric constant of greater than approximately 10.
Because of high direct tunneling currents, SiO2 films thinner than 1.5 nm generally cannot be used as the gate dielectric in CMOS devices. There are currently intense efforts in the search for the replacement of SiO2. Metal oxides, such as hafnium oxide (HfO2) and zirconium oxide (ZrO2), are leading candidates for high-k dielectric materials. The dielectric constant of these materials is about 20 to 25, which is a factor of 5-6 times that of silicon dioxide, meaning that a thickness of about 5-6 nm of these materials could be used to achieve an equivalent oxide thickness (EOT) of about 1.0 nm, assuming that the entire film is essentially composed of the high-k material. One problem with using high-k materials is that an interfacial layer of silicon dioxide, or a silicate layer, with a lower dielectric constant forms during standard processing. Achieving EOT of less than 1.5 nm is very difficult. An EOT of about 1.0 nm, and below, is expected to be used for the 0.07 micrometer device generation.
Deposition of ZrO2, or HfO2, using atomic layer deposition (ALD) tetrachloride precursors has been reported. Substrates heated to between 300° C. and 400° C. have been exposed to ZrCl4, or HfCl4, precursors alternating with water vapor in an attempt to form ZrO2 or HfO2 films respectively. However, it is difficult to initiate deposition on hydrogen terminated silicon surfaces. Hydrogen terminated silicon surfaces result from standard industry cleaning processes. These standard cleaning processes, which are often referred to as HF last clean, typically end in a quick dip in dilute HF. This produces surfaces which are hydrogen terminated, also known as hydrogen passivated. With sufficient exposure of the silicon surface to the reactants, the deposition may eventually be initiated. But, this results in films that are rough with poor uniformity. Another problem with tetrachloride precursors is the incorporation of residual chlorine in the film. The chlorine impurities can result in long term reliability and performance issues.
Other precursors use Hf or Zr metal combined with organic ligands such as iso-propoxide, TMHD (2,2,6,6-tetramethyl-3,5-heptanedionate), or combinations of organic ligands with chlorine. These precursors also have a problem initiating the film deposition on hydrogen terminated silicon surfaces and will incorporate carbon residues in the film. Large ligands may also take up enough space that steric hindrance will prevent the deposition of a uniform monolayer. Up until now, the successful implementation of ALD Zr and Hf oxides have been either on an initial layer of silicon oxide, silicon oxynitride, or in the form of a reduced dielectric constant silicate film, such as ZrSiO4 or HfSiO4. These initial layers may contribute significantly to the overall EOT.
The use of metal nitrate precursors, such as Hf(NO3)4 and Zr(NO3)4, has been shown to provide a means for depositing films onto hydrogen passivated surfaces, even at low temperatures required for ALD. Problems related to the interfacial layer, such as the formation of silicon oxides or silicates, may remain even using metal nitrate precursors. This may be, in part, due to the presence of H2O, during the formation of the interfacial layer, for example in a process employing alternating exposures, such as M(NO3)x/H2O (where M is a metal and x is the valence of M). The presence of H2O may affect the interfacial layer due to the presence of hydrogen, or due to the additional oxygen provided. Another possible cause is that the NO3 ligand is highly reactive and may promote the formation of silicate, or silicon oxide, at the interface even at moderate temperatures in the range of 300 to 450 degrees Celsius. Another factor may be that the amount of oxygen in the M(NO3)x in excess of that required to form the desired metal-oxide film, for example using Hf(NO3)4 (12:1 O:Hf ratio) to deposit HfO2 (2:1 O:Hf ratio).